How to Effectively Manage Phosphorus Levels in the Soil

Bound. Fixed. Unavailable.


Three words no grower wants to hear about one of the three most important nutrients in crop production. However, this adequately describes the nutrient known as phosphorous (P). While total soil P concentration is very high, it’s estimated that crops will seldom get more than 15%-20% of the P applied in fertilizer. Still, the overall trend in soil test P levels across the Midwest and Corn Belt continue a multi-decade long decline. This continual decline can be attributed to increasing yields coupled with P application rates that are below crop nutrient removal.

Knowing this, what advice can retailers give to growers to better effectively manage this critical nutrient? Phosphorus management and efficiency comes down to understanding the nutrient, each crop’s need, the soil properties, and making informed decisions on the type and rate of fertilizer used. Let’s take a deeper look.

An Essential Macronutrient

Phosphorus joins nitrogen and potassium as one of the three essential macronutrients needed in crop production. While phosphorus plays key roles in numerous plant processes, one of its primary functions is the storage and transfer of energy via ADP and ATP. Due to this dynamic relationship with energy, inadequate phosphorus levels will result in decreased cell division and impaired plant growth (stunting). In more severe deficiencies some plants can develop dark green or purplish colored leaves. This purpling is essentially due to a buildup of sugars that the plant doesn’t have enough energy to transport and utilize.

Phosphorus is also important for seedling and root growth and development. While phosphorus is needed in large quantities throughout the entire growing season, it’s especially critical early in a plant’s life. A young plant’s root system has limited capacity and its ability to obtain sufficient P can be relatively low, necessitating the need for proper P management. Therefore, it’s not uncommon to see the addition of P-based fertilizers being placed in close proximity to the developing roots.

It Starts With the Soil

With phosphorus being critical throughout a plant’s entire life cycle it’s important we understand how phosphorus’s availability is influenced by our soil. Soil phosphorus consists of two forms: organic (non-plant available) and inorganic (plant available) P. While these forms exist in equilibrium and replenish each other, the process is very slow, so the amount of available P can be relatively low. To add to the complexity, the inorganic, or plant available form of P, is highly reactive and can be tied up by our soils. Therefore, you’ll often hear the term P-solution used to describe the amount of inorganic P actually available to the plant. You may have heard the phrase “Plants don’t chew, they drink”, well this is exactly right as the inorganic P has to be dissolved into solution (P-solution) for the plant to be able to take it in. One of the problems we face when applying phosphorous fertilizers is its reactivity with our soils, as the majority of the P we apply will either adsorb to the soil or precipitate out of solution, both making it unavailable to our crops.

P Availability TableThe pH of the soil is also a significant driver with phosphorus as both acidic and basic soils influence its availability. Soil pH matters due to the solubility of different minerals within a soil. Phosphorus will bind and form insoluble precipitates (salts) with different soil micronutrients. Iron and aluminum, for example, are micronutrients whose solubility increases as the soil pH decreases, so the potential for unavailable salts to form increases as more iron and aluminum come into solution. Calcium, on the other hand, increases with increasing pH, so a more neutral pH of around 6.0-7.0 is ideal for maximum phosphorus availability.

Soil testing is one of the best tools for gauging nutrient levels in a field, and the results can be used as a guide for developing an effective phosphorus management plan. However, when it comes to phosphorus, most soil tests measure total extractable P, and not necessarily what will be available to the plant. Granted, a field testing low in phosphorus still indicates there is a high probability the crop will benefit from an application of phosphorus, but those soil properties like those previously discussed will influence how much P will be needed.

Increasing Nutrient Availability

Fertilizer applications are recommended to increase the concentration of plant available P the crop needs throughout the growing season. Because of phosphorus’s immobility, usually moving less than an inch from where it was applied, fertilizer placement can also be very important. This makes band and in-furrow applications an excellent strategy for growers worrying about their phosphorus levels.

Industry experts have documented that phosphorus applications are only about 20% efficient, further necessitating the need for proper P placement for maximum uptake and utilization. Placing P fertilizers in close proximity to the seed can increase the soil concentration more than a hundred fold, giving young seedlings a great head start, especially when planting into cooler soils. Further, timing of application is also important and the efficiency of P fertilizers is greatest when applying shortly before or at planting.

In addition, strongly consider using a fertilizer additive with a chelate on soils that are prone to phosphorus tie-up. The word “chelate” comes from the word claw and represents the way the molecule bends around a micronutrient to protect it. When chelated, these micronutrients will no longer tie up the phosphorus which, in turn, increases both phosphorus and micronutrient availability. Today the industry’s most powerful chelating agent for nutrient uptake is an ortho-ortho EDDHA chelate. It’s inclusion in products like Trivar, a broadcast fertilizer additive, has been shown to significantly enhance the availability of phosphorus in a soil, maximizing yields and providing a better return on investment.

For help with your nutrient management plan, contact your CHS Agronomy representative.


Leave a Reply

Avatar for George Rehm George Rehm says:

I believe that most statements are correct. However, there seems to be a paradox. One statement implies that phosphate fertilizer recommendations should be based on removal of phosphorus by plants. Yet other statements support soil testing as a predictor of needs. Soil testing does not measure the availability of nutrients in terms of pounds per acre. It’s important to remember that a soil test is a measure of the relative level of the nutrient in question. The use of soil testing for any nutrient has been researched for over40 years and there is no better analytical tool.